There are lots of SCR turn-off methods but the SCR turn-off method of this invention obtained after long study and experiment is quite different from the conventional ones. Take the SCR serving as a D.C. Switch for example. When the SCR gate sends out a turn-on pulse voltage, the SCR is on and the commutation capacitor is charged to store the electric energy needed for turn-off. When the pulse voltage of the SCR gate is zero, the circuits of this invention use the stored energy to turn off the SCR. The turn-off action is very fast, almost as soon as the sending of the SCR gate off signal. The SCR turn off action does not produce any sparks and thus it is very safe. From the above, it is seen that by applying the transient charge of voltage to the gate, this invention makes the SCR in turn off state and attains the objective of cutting off the source.